In a system where an output from a speaker may enter a microphone, such as in a hands-free calling system or a telephone conference system, it is required to secure call quality of the system, in particular, to cancel an echo caused by acoustic coupling occurring upon a two-way call.
For a conventional technique of canceling such echo, there is an acoustic echo canceller which cancels an echo from the speaker to the microphone by identifying an echo path by means of an adaptive filter that identifies a transfer function between a speaker and a microphone, and subtracting, from an input signal to the microphone, a pseudo-echo signal generated by using the identified echo path.
A general acoustic echo canceller cancels an echo by using a single adaptive filter. In the case where an impulse response of an echo path is longer, the adaptive filter is required to have a longer tap.
However, the adaptive filter has a drawback that the adaptive speed declines as the tap is longer.
For a conventional technique of solving the above-described drawback, for example, there is a Patent Literature 1 which discloses an echo canceller having a two-stage adaptive filter. In this echo canceller, in order to improve the speed of adaptive operation, an echo of a direct component is cancelled at a preceding stage and a reverberation component is removed at a subsequent stage. The adaptive filter provided at the preceding stage is shorten in tap length to remove the direct component of an echo. On the other hand, the adaptive filter provided at the subsequent stage is lengthened in tap length to remove the reverberation component that cannot be removed by the adaptive filter at the preceding stage. Since the direct component having a large proportion in an echo component is thus canceled at the preceding stage, an excellent amount of echo cancellation can be expected from the early stage of operation.
A Patent Literature 2 discloses a subband echo canceller that divides an acoustic signal band into a plurality of frequency bands and performs echo cancellation on the respective bands. The subband echo canceller divides an acoustic signal band and thins out an acoustic signal in each frequency band at a predetermined thin-out rate to generate subband signals, by which the sampling frequency can be reduced, enabling to reduce the amount of computation. This is because, when the sampling frequency becomes ½, the amount of computation by an adaptive filter for the same reverberation time is reduced to ¼.